It is desirable to limit the production of NO.sub.x in fuel oil combustors. In the art of gas turbines, there is existing technology that achieves low NO.sub.x combustion by the lean premixing of gaseous fuel with air. One such technique produces a sufficiently dilute fuel/air mixture to reduce the rate of NO.sub.x formation below the rate which exists in a combustion zone having the stoichiometrically correct fuel/air ratio. In another technique disclosed in U.S. Pat. No. 4,089,639 issued May 18, 1978 to Reed et al., water vapor is premixed with a vaporized fuel prior to the burning operation to produce reduced levels of NO.sub.x.
In order to produce a lean or dilute fuel/air mixture with fuel oil, the fuel oil must be in the form of a vapor intimately mixed with air, or with air and inerts, so that the ratio of available fuel oil to air and/or inerts is low. When such a vaporized fuel oil is utilized in a combustor, the mass of the air and/or inerts intimately mixed with and surrounding the fuel oil acts as a large heat sink at the molecular site of combustion. This heat sink absorbs the heat of combustion without raising the temperature of the combustion products and the air and inert substances intimately mixed with the combustion products beyond the point where rapid NO.sub.x formation occurs.
The challenge to achieving such a dilute mixture with fuel oil is that the oil is normally introduced into a gas turbine combustor as a finely divided or atomized spray of small droplets. The space around the oil spray is fuel-rich and there is not necessarily enough time for all the oil droplets to evaporate completely and/or mix uniformly with a large volume of air prior to initiation of combustion. The combustion taking place in this fuel-rich volume, particularly at the surface of unvaporized oil droplets and in the region where there is a fuel rich oil vapor to air mixture, is at or near the stoichiometrically correct air/fuel ratio and produces a corresponding local adiabatic flame temperature high enough to produce a rapid rate of thermal NO.sub.x formation. Unfortunately, attempting to pre-vaporize the fuel oil prior to its introduction into the combustor by providing its heat of vaporization from a heat source having a temperature high enough to cause oil carbonization, risks carbonization of the oil onto hot heat transfer surfaces or premature combustion of the liquid and/or vapor.
In some gas turbines burning oil, compressed air from atomizing air compressors provides the mechanical energy to break the liquid fuel oil up into small droplets at the fuel nozzle exit. Steam atomizers have been used in boilers to provide mechanical energy for breaking preheated heavy oil up into small droplets for introduction into the furnace of the boiler.
A heat source for pre-vaporization of fuel oil is required that has a high heat capacity and heat flux so as to produce a sufficiently high temperature for causing rapid and complete vaporization of an atomized oil spray, yet has a temperature sufficiently low so as to prevent carbonization of the fuel oil.